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Exploiting your antenna analyser #12

Is there a place for UHF series connectors in critical measurement at UHF?

Seeing some recent discussion by a chap who was trying to construct a low power 50Ω termination on a UHF series plug, it bought to mind the futility of using some kinds of connector for critical measurement above perhaps 100MHz.

There is a lot of conjecture about the nature of UHF series connectors, whether they act line a simple transmission line section with fairly uniform Zo, whether they are really just a lumped shunt capacitance, whether it is even important at UHF etc.

To illustrate the issue, I have assembled a simple test jig comprising an N(M)-UHF(F) adapter, UHF(M)-N(F) adapter and a 50Ω N termination (which was also used to calibrate the analyser. This set was assembled and plugged onto a calibrated AIMuhf analyser and swept from 1-500MHz… just into the UHF range (which is 300-3000MHz).

Above, the test jig.

Above is an expanded scale centre of the Smith chart of the sweep.

We clearly have a near circular arc, and visually it can be seen to be centred on about Z’=0.8+j0Ω (Z=40+j0Ω). This strongly hints that there is a transmission line between the analyser port and the 50Ω termination, and that Zo of that line is around 40Ω.

Above is a plot of VSWR after adjusting Zref for the flattest VSWR response… and it is very flat which suggests that the first impression of some line of fairly constant Zo is a good model. This plot is quite sensitive to Zref, and we can come to a value of 38.4Ω.

So, the pair of adapters behaves very much like a transmission line section of Zo=38.4Ω, and the observed impedance transformation at 300MHz suggests it is 55mm in electrical length, an average of 27.5mm for each adapter and allowing for the use of solid dielectic, this accounts for their physical length of around 23mm.

Now the futility of building a low VSWR 50Ω termination onto the back of a UHF plug becomes apparent, the inclusion of a length, albeit short, of 38Ω transmission line causes degradation of VSWR at higher frequencies… in the VHF range, much less UHF.